Source for replacement caps in H744 regulators

Brent Hilpert bhilpert at
Thu Jan 6 19:28:44 CST 2022

On 2022-Jan-06, at 4:04 PM, Matt Burke via cctalk wrote:
> On 06/01/2022 12:59, Jonathan Chapman via cctalk wrote:
>> That said, it's not like replacing them with new will *hurt* -- it just might not fix the whine.
> I suspect that it won't fix the problem. Slightly hijacking the thread
> here but hopefully in a semi-helpful way.
> I have two of these regulators, one from a BA11K and one from a TS11.
> Under the same test conditions the one from the TS11 produces a
> significantly louder whining noise than the one from the BA11K. Given
> that they are largely the same circuit as the H745 regulators and all
> the ones I have are silent in operation I find it hard to believe they
> are supposed to be like this. I used to have two more TS11 drives and
> they both exhibited the same behaviour. Seems to be a common problem.
> Now, for the one from the BA11K (quiet one) the output capacitors were
> completely open (0uF) so I had to replace them. Given that I had the new
> capacitors I decided to try them in the TS11 regulator to see if it
> would fix the whining noise. It made no difference.
> After some probing with an oscilloscope I found that between the two
> regulators there was a very different waveform on the emitter of Q2.
> Quiet regulator:
> Noisy regulator:
> The yellow trace is the emitter of Q2 (input of L1) and the cyan trace
> is the output of L1. Q2 seems to be switching partly on then fully on in
> the noisy regulator. it should of course be fully on or fully off as
> seen on the quiet regulator. Also the switching frequency seems to be lower.

The intermediate step voltage at Q2.E-L1.input is not Q2 partially on. It's D5 shutting off, as it would do when L1 has discharged and current is no longer flowing in it, and follows from the basic principles of a buck converter. Notice that the step is 5V, i.e. the output voltage.

> I tried testing/swapping a few parts between the two regulators, L1, Q2,
> Q3, E1. It started out reasonably logical and after several days
> descended into a unscientific mess of swapping anything and everything
> that could possibly be at fault. I got to the point where I had
> eliminated just about every component so I must have overlooked
> something. I was hoping to stumble upon the answer and then learn
> something from it but so far no luck. This project has been shelved for
> a while now.

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